Science uncovers more details about understanding sense of smell

Our sense of smell has a lot to do with who we are. The average human nose can pick up 10,000 different odors. It accounts for 80 percent of what we taste.

So powerful is our sense of smell that we can detect a skunk from 0.000.000.000.000.071 of an ounce of scent, according to the Fragrance Foundation's Sense of Smell Institute in New York City.

But despite this high level of sensitivity, smell ability is frequently disrupted by nasal congestion, a cold or other virus, allergies and other conditions.

The sense also worsens with age. About 2 percent of all people have olfactory dysfunction to some extent, but studies suggest that number rises to 25 percent of men and 11 percent of women between the ages of 60 and 69.

Long-term blockage of the sense of smell may require surgery or other interventions, but most disruptions last only a week or so, and the scent detection system generally picks right back up, none the worse for wear.

Researchers at Northwestern University's School of Medicine studied the brain changes that occurred among 14 people whose noses were stopped up most of each day for a week and slept in a special low-odor hospital room.

They reported in the August issue of Nature Neuroscience that the two regions related to the sense of smell went into a sort of "safe mode" while the scent-sensors high in the nose were deprived. One region became more active, the other less.

Within a week of restoring the subjects to unrestricted breathing, the brain's response to odors had returned to levels similar to what was seen before the experiment.

Another study, done by researchers at the University of Michigan, showed that they could genetically restore function to mice with a mutation that had cost them their sense of smell.

The scientists reported in the journal Nature Medicine in early September that they used a version of a common cold virus to deliver a gene sequence the mice were missing. When working normally, the gene creates a protein needed for the development of cilia -- antenna-like projections on sensory cells that help them sense what is around.

"Essentially, we induced the neurons that transmit the sense of smell to regrow the cilia they'd lost," said Jeffrey Martins, an associate professor of pharmacology, in a statement about the study, which was sponsored by the National Institutes of Health.

The mice got the therapy for three days, and then had 10 days to allow the protein to grow. Two weeks later, researchers could tell that neurons connected to smell were working, and the mice, which had lost much of their weight due to the lack of taste, were eating more.

One region that guides the sense of smell lies in the frontal cortex. A study by Australian scientists published Sept. 20 in the journal Chemosensory Perception showed that people who have psychopathic traits are also likely to have an impaired sense of smell.

Their work, which involved 79 people, first measured the olfactory ability as well as the sensitivity of each person's sense of smell. Then, they tested each person for manipulation, callousness, erratic lifestyles and criminal tendencies, and looked at how much or little they empathized with others feelings.

They found that those individuals who scored high for psychopathic traits had more trouble identifying smells and telling the difference between smells, even though they knew they smelled something. The researchers note that not everyone with psychopathic traits is necessarily criminal, but that the lack of sense of smell may be a marker for eccentric, psychopathic traits found in many with the personality disorder who are nonetheless highly successful.